1. Field of the Invention
The present invention relates to a semiconductor laser used as a light source for optical information processing.
2. Description of the Prior Art
Long-service-life semiconductor lasers with small changes in threshold current upon changes in temperature are required as light sources for optical information processing. To prolong the service life, it is effective to decrease the threshold current of a laser. For these reasons, semiconductor lasers having quantum well structures as active layers are used as the light sources. In particular, a strained quantum well structure in which a semiconductor constituting a well layer having a lattice constant different from that of a substrate has been known to be effective.
The reliability of a laser is improved by shifting the lattice constant of a barrier layer in a direction opposite to a well layer, and compensating the strain of the whole quantum well.
To increase the information storage capacity of optical disks and the like, short-wavelength lasers are required. Such a laser can be attained by using GaAsP with a lattice constant smaller than that of conventional GaAs for a substrate, which is reported in H. Kressel, C. J. Nuesse, and G. H. Olsen, "Journal of Applied Physics", Vol. 49, pp. 3140 -3149, 1978, and T. Tanaka, K. Uchida, Y. Ishitani, and S. Minagawa "Applied Physics Letters", Vol. 66, pp. 783-785, 1995. In these references, the wavelength of an active layer lattice-matched with a substrate is shortened by utilizing a semiconductor crystal with a small lattice constant having a large band gap.
When the characteristics of a semiconductor laser for emitting a beam with wavelength around 635 nm are improved by introducing the above-described strained quantum well structure into the semiconductor laser, the following problems arise.
First, in a compressive-strained quantum well structure wherein a well layer has a lattice constant higher than that of a substrate, a very thin well layer with a thickness of 1 to 2 nm must be formed to emit a beam around 635 nm because the band gap of the material for the well layer itself is small. Accordingly, the characteristics are degraded due to variations in thickness of the well layer in growth.
Second, in a tensile-strained quantum well wherein a well layer has a lattice constant smaller than that of a substrate, the well layer can be made thick because the band gap of the material for the well layer is large. However, carrier confinement is degraded because a difference in band gap with respect to a barrier layer becomes small. Leakage of carriers from an active layer degrades the threshold current and temperature characteristics. Particularly in a strain compensation structure wherein a compressive strain is applied to a barrier layer, almost no potential barrier with respect to electrons exists between a well layer and the barrier layer, resulting in an increase in leakage current.
Third, if the Al composition of AlGaInP serving as a barrier layer is increased in order to increase a band gap between the well and barrier layers, the band structure changes to be of an indirect transition type. For this reason, energy at the X point decreases to increase leakage carriers passing through the X point.
As described above, many problems arise in forming a 630-nm-band laser with a strained quantum well active layer, and the performance is difficult to improve.
Next, the problems of a semiconductor laser using a strain compensated tensile-strained quantum well for compensating the strain of the whole quantum well by shifting the lattice constant of a barrier layer in a direction opposite to that of a well layer will be described in detail.
FIG. 1A is a schematic view showing a semiconductor laser using a strain compensated tensile-strained quantum well. A cladding layer 12, a guide layer 13, an active layer 14, a guide layer 15, and a cladding layer 16 are sequentially stacked on a substrate 11.
As materials for the respective layers, GaAs is used for the substrate 11; (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P lattice-matched with the substrate, for the cladding layers 12 and 16; (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P lattice-matched with the substrate, for the guide layers 13 and 15; and Ga.sub.0.64 In.sub.0.36 P with a strain of 0.93% and Al.sub.0.42 In.sub.0.58 P with a strain of -0.7%, respectively, for a well layer 141 and a barrier layer 142 in the quantum well structure of the active layer 14.
FIG. 1B is a diagram of the conduction band edge showing energy at the .GAMMA. points and energy at the X points of the respective layers from the well layer to the cladding layer in the semiconductor laser using the strain compented tensile-strained quantum well.
With the materials used in FIG. 1A, the energy of electrons at the .GAMMA. point of the well layer 141, and the energy of electrons at the .GAMMA. points of the barrier layer 142 and the guide layer 13 have almost no difference, as shown in FIG. 1B. Only the cladding layer 12 has an energy difference of 30 meV. Therefore, the quantum well effect cannot be expected, and a leakage current is very large.
Further, energy differences between the .GAMMA. point of the well layer 141 and the X points of the barrier layer 142 and the cladding layer 12 are only 110 meV, and the number of leakage carriers passing through the X points is large. For this reason, if the Al compositions of the barrier layer 142, the guide layer 13, and the cladding layer 12 are increased to make energy differences at the .GAMMA. points large, energy at the X points decreases, resulting in further increases in leakage carriers passing through the X points. Therefore, the overflow of carriers cannot be prevented.
As described above, when a 630-nm-band laser is formed with the tensile-strained quantum well active layer, the performance is difficult to improve because the size of the strain cannot be increased.
When the wavelength is shortened by using GaAsP with a lattice constant smaller than that of a conventional GaAs substrate, the composition of a GaInP active layer to be lattice-matched becomes close to GaP. For this reason, leakage carriers from the X point increase, and the characteristics cannot be improved. Therefore, it is limited to shorten the wavelength by decreasing the lattice constant of the substrate.